1. Field of the Invention
The present invention relates to a lens device and, more particularly, to a lens device in which a first movable lens holding frame and a second movable lens holding frame are moved forward or rearward along an optical axis by rotating a cam cylinder to perform zooming.
2. Description of the Related Art
In zoom lens devices for electric news gathering (ENG) cameras, a type of zoom lens device is dominant in which zooming is performed by two movable lens group: a first movable lens group functioning as a variable-power lens group, and a second movable lens functioning as a focus correcting lens group. A cam cylinder is rotatably provided in the lens barrel of such a zoom lens device. In the cam cylinder are formed cam channels engaged with cam rollers projecting from a frame for holding the first movable lens group (hereinafter referred to as “first movable lens holding frame”) and a frame for holding the second movable lens group (hereinafter referred to as “second movable lens holding frame”). These cam rollers are passed through the cam channels to be engaged with straight-travel grooves formed in a fixed cylinder placed outside the cam cylinder. When the cam cylinder is rotated, the first and second movable lens holding frames are moved forward or rearward in predetermined paths along the optical axis by the straight-travel guide function of the cam channel and the straight-travel grooves, thereby adjusting the focal length.
On the other hand, a zoom lens device is known which is designed to reduce its overall size by reducing the lens optical path length while maintaining the desired image-taking power. In this zoom lens device, when the distance between the first and second movable lens holding frames is minimized, escape channels for escapement of the cam rollers on the second movable lens holding frame are formed in the first movable lens holding frame to set a longer stroke of movement between the first and second movable lens holding frames, thereby ensuring the desired image-taking power.
In the case where such escape channels are formed in the first movable lens holding frame, there is no problem when the cam rollers on the second movable lens holding frame are positioned in the escape channels. However, when the cam rollers are retreated from the escape channels, detrimental rays (stray light) enter the camera through the escape channels to generate ghost.
To solve this problem, a light shielding member for closing the escape channels of the first movable lens holding frame is provided in a lens device disclosed in Japanese Patent Application Laid-Open No. 2003-248157. The light shielding member prevents detrimental rays from entering the camera through the escape channels. When the distance between the first and second movable lens holding frames is minimized, the light shielding member is retreated from the escape channels by being forced by the cam rollers on the second movable lens holding frame. When the cam rollers are retreated from the escape channels, the light shielding member is returned by an urging force of a spring to the position at which it closes the escape channels.
In the lens device disclosed in Japanese Patent Application Laid-Open No. 2003-248157, however, impact sound and vibration are generated when the cam rollers on the second movable lens holding frame collides against the light shielding member. A cameraperson operating the lens device has a feeling of strangeness from the impact sound and vibration.